Some users are reporting disappearing Eridium-tier backpack and bank SDU upgrades. A patch was released on March 23, 2015 which addresses this issue. After the update, some players are reporting that their backpack slots are maxed out at 36 instead 39. We are currently investigating possible fixes for this issue. If you have any questions, please submit a support request at http://support.gearboxsoftware.com.